Generally, a wind turbine generator includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform mechanical wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators are generally, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbine generators also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
Some wind turbine generator configurations include doubly fed induction generators (DFIGs). Such configurations may also include power converters that are used to transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Alternatively, some wind turbine configurations include, but are not limited to, alternative types of induction generators, permanent magnet (PM) synchronous generators and electrically-excited synchronous generators and switched reluctance generators.
These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
Similar to wind generation, solar power generation is becoming a progressively larger source of energy throughout the world. Solar power generation systems typically include one or more photovoltaic arrays (PV arrays) having multiple interconnected solar cells that convert solar energy into DC power through the photovoltaic effect. In order to interface the output of the PV arrays to a utility grid, a solar converter is needed to change the DC current and DC voltage output of the PV array into a 60/50 Hz AC current waveform that feeds power to the utility grid.
Various solar power converters exist for interfacing the DC output of a PV array into AC power. One implementation of a solar power converter consists of two stages, a boost converter stage and an inverter stage. The boost converter controls the flow of DC power from the PV array onto a DC bus. The inverter converts the power supplied to the DC bus into an AC current and AC voltage that can be output to the AC grid.
In some instances, electrical machines like sources of electrical generation such as the wind turbine generators or photovoltaic arrays described above may be located in remote areas far from the loads they serve. Typically, these sources of generation are connected to the electrical grid through an electrical system such as long transmission lines. These transmission lines are connected to the grid using one or more breakers. In some instances, a grid event can occur on these electrical systems. Such grid events may cause high voltage events, low voltage events, zero voltage events, frequency shifts, phase shifts and the like, that may detrimentally affect the one or more electrical machines if protective actions are not taken. In some instances, these grid events can be caused by opening of one or more phase conductors of the electrical system resulting in islanding of at least one of the one or more electrical machines. Islanding of these electrical machines by sudden tripping of the transmission line breaker at the grid side or otherwise opening these transmission lines while the source of generation is under heavy load may result in an overvoltage on the transmission line that can lead to damage to the source of generation or equipment associated with the source of generation such as converters and inverters. Islanding generally requires disconnecting at least a portion of the affected one or more electrical machines from the electrical system to prevent damaging the electrical machine or equipment associated with the electrical machine. However, in other instances, the grid fault may not be islanding and may be a short term aberration to the electrical system. In these instances, it is desirous to keep the affected electrical machines connected to the electrical system and to institute ride-through procedures such as, for example, high voltage ride through (HVRT), low voltage ride through (LVRT) and zero voltage ride through (ZVRT). Exemplary systems and methods for HVRT, ZVRT and LVRT are described in U.S. Patent Publication U.S. 20120133343 A1 (application Ser. No. 13/323,309) filed Dec. 12, 2011; U.S. Pat. No. 7,321,221 issued Jan. 22, 2008; and U.S. Pat. No. 6,921,985 issued Jul. 26, 2005, respectively, which are fully incorporated herein by reference and made a part hereof.
Failure to properly detect and manage the occurrence of islanding events in wind turbines, photovoltaic arrays, or other power generator systems can be very damaging to those systems, especially when the power generation system is using a doubly fed induction generator typology. When an upstream breaker opens and leaves the wind farm, photovoltaic array or other power generation system isolated from the grid, the ac voltage seen by the power generation source can reach dangerous levels within a few milliseconds. This high ac voltage is more extreme on systems where the remaining connection to the grid has substantial length of power lines that are seen as a shunt capacitance. The event also has potential for a higher degree of damage as the power output of the power generation source increases, for instance, if they are in an overload condition during high winds. In many instances, detection of a grid event, such as islanding, depend on injecting a signal into the grid that would tend to cause a disturbance in some parameter of the grid (such as a current, voltage, frequency, or phase), and measuring the amount this disturbance has on some parameter (such as a current, voltage, frequency, or phase). It is desirable to find a new method of islanding detection without injecting a special voltage, current, phase, or frequency disturbance into the grid.
Accordingly, an improved system and/or method that provides for detecting a grid event without injecting a signal into the grid would be welcomed in the technology.